Nickel-base superalloys are used in gas turbine engine components because of their superior creep strength properties in high temperature operating conditions. While such single-crystal superalloys represent a substantial improvement over other prior known nickel-base superalloys that do not have a directionally oriented microstructure, they represent a compromise with respect to environmental attack during use, particularly to hot corrosion caused by oxidation and sulfidation. Further, despite the increased strength, single-crystal alloys are susceptible to cracks or other types of damage or deterioration that can occur from such experiences from thermal cycling or airborne object impact, or their combinations. Also, discontinuities such as shrinkage, inclusions and cracks can occur during the manufacture of such components. Because of the relatively high cost of such components, it is sometimes desirable to repair rather than to replace them. However, superalloy components, once damaged, can tend to fail repeatedly in the same region, making it critical that any repairs made have mechanical, environmental, and processing properties equivalent to or better than the original superalloy base metal.
Methods are known in the art to repair certain nickel-base superalloys, such as wide-gap brazing or transient-liquid phase processes. These processes involve cleaning the turbine engine component, applying a compatible diffusion bond mixture having a melting temperature lower than that of the substrate, and heating the article to melt the diffusion bond mixture without causing thermal damage to the substrate. Most of these processes are used on equiaxed alloys, but it has been found to be more difficult to repair single-crystal alloys because introduction of impurities to the unicrystal can create points of weakness.
Repair diffusion bond mixtures have been formulated to include substantial amounts of melting point depressants, primarily silicon and boron, so that the diffusion bond mixture can be applied at temperatures that are sufficiently high to melt the mixture, but not high enough to melt the substrate and cause thermal damage to the substrate. Because the amount of melting point depressants is so high, however, the diffusion bond mixture may differ significantly in composition from the substrate and the repaired area may not have the same properties as the adjacent substrate, increasing the risk of failure at the repaired area. In particular, undesirable large blocky or script-like brittle phases composed of chromium, titanium, and the family of refractory elements (e.g., tungsten, tantalum) combined with the melting point depressants grow in the eutectic that forms between the substrate and the diffusion bond mixture. These brittle phases may weaken the repair composite. It should be appreciated, however, that conventional repairs generally result in safe end products when properly used.
Therefore, there is a need for a composition and method that minimizes the amount of diffusion bond mixture alloy applied to the part, and therefore the amount of boron. There is also need for a composition and method that minimizes eutectic brittle borides common to wide gap brazing processes and a composition and method to repair superalloy substrates that results in a repaired area that has properties very similar to the substrate.